Optical transceivers are utilized in a number of systems to transmit and receive data and to implement proximity detectors. Such devices typically include a light source, which is typically a light emitting diode (LED) that is used to transmit data by modulating the intensity of the light source and a photodiode that receives the modulated light signals. Optical transceivers operating in the infrared are utilized in computers and handheld devices for transferring data from one device to another without requiring that the devices be connected together by a wire or cable. In such systems, the two devices are positioned relative to one another such that light from the transmitter in the first device is received by the optical receiver in the second device, and vice versa.
In a proximity detector, the light from a transmitter in the transceiver is received by the receiver in that transceiver after the light has been reflected from the surface of an object that is being detected. The amount of light that is received by the receiver is a function of the surface properties of the object and the distance between the object and the transceiver. Such proximity detectors are utilized in handheld devices such as cellular telephones to adjust the amplifier levels in response to the user placing the device close to the user's face.
The light levels received by the photodiode in the receiver can be quite small. The beam profile from the transmitter is spread by the reflecting surface which is typically a surface having a low reflectivity that introduces a significant degree of scattering into the optical path. Hence, the receiver sees a beam having a cross-section that is much larger than the photodiode. As a result, the fraction of the light that enters the receiver is only a small fraction of the light that left the transmitter. Hence, the signal-to-noise ratio in the receiver can be quite small.
To improve the signal-to-noise ratio, the collection angle of the receiver is increased by providing a lens that collects light over an area that is much larger than the photodiode and then focuses that light on the photodiode. In addition, a collimating lens can be provided over the transmitter to shape the profile of the illumination source to provide a smaller beam cross-section at the receiver. While these lenses improve the signal-to-noise ratio, they increase the height of the transceiver.
However, in many applications, there is a limit to the size of the transceiver. Many handheld devices such as cellular telephones fall into this category. Hence, this solution is not always viable when a small transceiver is required.
In addition, the lenses can introduce cross-talk into the transceiver. Light from the transmitter is reflected off of the lens-air surfaces and reaches the receiver. Since the signal-to-noise ratio is already compromised by the small fraction of the light reflected by the object of interest that reaches the receiver, this cross-talk can pose a significant problem. To reduce the cross-talk, additional light blocking baffles must be introduced between the receiver and transmitter. Such baffles further increase the cost and size of the transceiver.